Flexible stent

ABSTRACT

A preferred embodiment of a stent provides a folded strut section that provides both structural rigidity and reduction in foreshortening of the stent mechanism. A flexible section provides flexibility for delivery of the stent mechanism. In a second embodiment, flexible section columns are angled with respect to each other, and to the longitudinal axis of the stent. These relatively flexible sections are oppositely phased in order to negate any torsion along their length. In yet another embodiment, the flexible connector can take on an undulating shape (like an “N”), but such that the longitudinal axis of the connector is not parallel with the longitudinal axis of the stent. Finally, a new method is disclosed for making stents. The method consists of performing a standard photochemical machining process of cutting, cleaning and coating the tube with a photoresist. However, unlike former methods, the photoresist image is developed on the surface of the cylindrical metallic tube, which results in a controlled variable etching rate at selected sites on the cylindrical metallic tube during the etching process. Further embodiments provide living hinge connectors and connections along the length of the radial strut member.

BACKGROUND ART

[0001] A stent is commonly used as a tubular structure left inside thelumen of a duct to relieve an obstruction. Commonly, stents are insertedinto the lumen in a non-expanded form and are then expanded autonomously(or with the aid of a second device) in situ. A typical method ofexpansion occurs through the use of a catheter mounted angioplastyballoon, which is inflated within the stenosed vessel or bodypassageway, in order to shear and disrupt the obstructions associatedwith the wall components of the vessel and to obtain an enlarged lumen.

[0002] In the absence of a stent, restenosis may occur as a result ofelastic recoil of the stenotic lesion. Although a number of stentdesigns have been reported, these designs have suffered from a number oflimitations. These include restrictions on the dimension of the stent.

[0003] Other stents are described as longitudinally flexible but consistof a plurality of cylindrical elements connected together. This designhas at least one important disadvantage, for example, according to thisdesign, protruding edges occur when the stent is flexed around a curveraising the possibility of inadvertent retention of the stent on plaquedeposited on arterial walls. This may cause the stent to form emboli ormove out of position and further cause damage to the interior lining ofhealthy vessels.

[0004] Thus, stents are known in the art. Such stents may be expandedduring or just after balloon angioplasty. As a general rule, themanufacture of a stent will need to compromise axial flexibility inorder to permit expansion and provide overall structural integrity.

[0005] Prior stents have had a first end and a second end with anintermediate section between the two ends. The stent further has alongitudinal axis and comprises a plurality of longitudinally disposedbands, wherein each band defines a generally continuous wave along aline segment parallel to the longitudinal axis. A plurality of linksmaintains the bands in a tubular structure. In a further embodiment ofthe invention, each longitudinally disposed band of the stent isconnected, at a plurality of periodic locations, by a shortcircumferential link to an adjacent band. The wave associated with eachof the bands has approximately the same fundamental spatial frequency inthe intermediate section, and the bands are so disposed that the wavesassociated with them are spatially aligned so as to be generally inphase with one another. The spatial aligned bands are connected, at aplurality of periodic locations, by a short circumferential link to anadjacent band.

[0006] In particular, at each one of a first group of common axialpositions, there is a circumferential link between each of a first setof adjacent pairs of bands.

[0007] At each one of a second group of common axial positions, there isa circumferential link between each of a second set of adjacent rows ofbands, wherein, along the longitudinal axis, a common axial positionoccurs alternately in the first group and in the second group, and thefirst and second sets are selected so that a given band is linked to aneighboring band at only one of the first and second groups of commonaxial positions.

[0008] Furthermore, this stent can be modified to provide for bifurcatedaccess, whereas the stent itself is uniform throughout. If themanufacturer designs such a stent to have an large enough opening, thenit is possible to place the stent such that a pair of stents can beplaced one through the other. In this fashion, the stents are capable ofbeing placed at a bifurcation, without any welding or any specialattachments. An interlocking mechanism can be incorporated into thestent design to cause the stent to interlock at the desired positionduring assembly of the device.

[0009] Further, a metallic stent has been designed which contains arepeating closed loop feature. The stent is designed such that theclosed loop does not change dimensions during expansion. The compositestent is created by filling the area enclosed by the loops with amaterial that enhances clinical performance of the stent. The materialmay be a ceramic or a polymer, and may be permanent or absorbable,porous or nonporous and may contain one or more of the following: atherapeutic agent, a radio-opaque dye, a radioactive material, or amaterial capable of releasing a therapeutic agent, such as rapamycin,cladribine, heparin, nitrous oxide or any other know drugs, either aloneor in combination.

[0010] It has been seen, however, that it may be desirable to providefor stents that have both flexibility to navigate a tortuous lesion aswell as increased column strength to maintain the rigidity necessaryafter emplacement into the lumen of the body. The preferred designs tendto provide the flexibility via undulating longitudinal connectors. Therigidity is generally provided via the mechanism of slotted tubularstents. It is perceived that there may be mechanisms capable ofenhancing the characteristics of these types of stents. Such a stentwould be both flexible in delivery and rigid upon emplacement.

[0011] Furthermore, it is desirable to be able to produce stents inwhich the cross-sectional profile of either the struts or the connectingmembers is tapered (or variable) in size. In addition, it may bedesirable to modify stents to have non-rectangular cross-sections. Inboth these cases, different manufacturing methods may aid in thecreation of such stents.

SUMMARY OF THE INVENTION

[0012] It is an object of the invention to provide a stent having hasrelatively little foreshortening.

[0013] It is an object of the invention to provide a stent having anenhanced degree of flexibility.

[0014] It is an object of the invention to provide such a stent whilediminishing any compromise in the stent's structural rigidity uponexpansion.

[0015] It is a further object of the invention to provide a novel methodfor manufacturing stents.

[0016] These and other objects of the invention are described in thefollowing specification. As described herein, a preferred embodiment ofa stent provides for a device that contains a flexible section and afolded strut section. The folded strut section opens (like a flower)upon expansion. This folded strut section provides both structuralrigidity and reduction in foreshortening of the stent mechanism. Theflexible section provides flexibility for delivery of the stentmechanism.

[0017] In a second embodiment of the device, there is a columnar sectionand a flexible section. The columnar section provides for a device thatlengthens in the longitudinal direction upon expansion. The flexiblesection provides for a section that shortens somewhat in thelongitudinal direction upon expansion. As a result, there is noshortening or lengthening of the stent during expansion. The flexiblesection columns are angled, one with respect to the other, and also withrespect to the longitudinal axis of the stent, in order to provideflexibility during delivery. This arrangement also to also provideadditional resistance to the balloon to prevent “dogboning” of the stenton the balloon during delivery and slippage of the balloon along thestent. These relatively flexible sections are oppositely phased withrespect to one another in order to negate any torsion along theirlength. These flexible sections can further be crimped onto the ballooncatheter with a generally smaller profile than prior stent, so that theretention of the stent on the balloon is increased.

[0018] In yet another embodiment of the stent of the present invention,the flexible connector can take on an undulating shape (like an “N”),but such that the longitudinal axis of the connector is not parallelwith the longitudinal axis of the stent. In this fashion, theflexibility is controlled in a pre-selected axis, which is not thelongitudinal axis of the stent. Such an arrangement may be desired, forinstance, when one chooses to place a stent in a particularly configuredvasculature that has been predetermined by known means, such asintravascular ultrasound (“IVUS.”)

[0019] In still a further embodiment of the present invention, there areprovided “living hinge” connectors, which connect the generally flexibleconnectors to the stronger radial strut members. These living hingesaccomplish a number of the same characteristics found in the priorembodiments disclosed herein. First, because the living hinges tend toexpand upon inflation, foreshortening of the length of the stent isfurther reduced. Second, there is a combined radial strength provided atthe intersection between the living hinges and the radial strut members.This creates a small “hoop,” which is further resistant to kinking orcollapse in situ. Third, as a corollary to the second attributedescribed above, the living hinge connectors provide for reduced strainalong an equivalent length of stent.

[0020] In yet another preferred embodiment of the stent of the presentinvention, the connection point between the radial members and theconnector members is moved to a position along the length of a radialstrut. Typically, the connection may be placed at a position somewheremidway along the length of the strut. By moving the connection point ofthe flex connectors closer to the midpoint of the radial ring one canaddress foreshortening in an controlled fashion. In fact, ballooninteraction aside, the connector does not have to stretch to compensatefor foreshortening. When the flex connectors are connected at themidpoint of the radial ring, the distance/length through the middleportion of the stent between radial rings will remain unchanged. This isbecause the midpoint stays relativiely in the same position while theradial arcs of each strut move closer to the midpoint from both sides.By moving the location of the flex connector attachment beyond themid-point of a strut, to the opposing side, one can actually capitilizeon the strut moving closer to the midpoint and thus lengthen the stentupon expansion.

[0021] In addition, in the present embodiment described, adjacentradially rings start out of phase in the unexpanded state. Due to thediagonal oreintation of the connection points of the flexibleconnectors, upon expansion the radial rings tend to align themselves(“in” phase.) This results in more uniform cell space and thus improvedscaffolding of the vessel. Further, there is described a “wavy” strutconfiguration, thereby facilitating both a reduced crimp profile forattaching the flexible connectors at or near a strut mid-point andreduced strain upon expansion, due to the strut itself contributing to aportion of the expansion.

[0022] Finally, a new method is disclosed for making stents. In thismethod there is novel photochemical machining of a cylindrical tube. Themethod consists of performing a standard photochemical machining processof cutting, cleaning and coating the tube with a photoresist. However,unlike former methods, the photoresist image is developed on the surfaceof the cylindrical metallic tube, which results in a controlled variableetching rate at selected sites on the cylindrical metallic tube duringthe etching process. The photoresist image consists of a series ofcircular regions of photoresist of varying diameters configured atvarying distances along the stent. As the diameter of the circularphotoresist pattern decreases and the distance between the circularphotoresist patterns along the stent increases, the etch rate of thedevice increases. The photoresist pattern variation results in avariation in the metal removed during the etching process.

[0023] This process can be used to locally change the geometry of thecylindrical metallic tube. An advantage seen by this process is theability to manufacture a tapered strut along the stent. Further, strutsof cylindrical or other non-rectangular cross-section can bemanufactured. In addition, surface contours can be placed on the stent,for instance, to allow for a reservoir to be placed in the stent todeliver drugs.

[0024] These and further objects of the invention will be seen from thefollowing drawings and Detailed Description of the Invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a plan view of a stent embodying the invention;

[0026]FIGS. 2 and 3 are plan views of an alternative embodiment of astent of the invention;

[0027]FIG. 4 is a plan view of yet another embodiment of a stent of theinvention;

[0028]FIG. 5 is a close up of the identified section of FIG. 4 takenalong lines b-b of FIG. 4;

[0029]FIG. 6 is a schematic of a photoresist pattern formed on the stentin order to perform a method for making the stent as described in theinvention;

[0030]FIG. 7 is a plan view of yet another alternate embodiment of thepresent invention;

[0031]FIG. 8 is a plan view of a further alternate embodiment of thepresent invention; and

[0032]FIGS. 9 and 10 are schematics of the theory behind expansion ofthe stent of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

[0033] As can be seen in FIG. 1, there is described a cylindrical stent10 which has a series of folded strut sections 20 connected by a seriesof flexible sections 30. The folded strut sections 20 comprise agenerally folded strut member 25 having a pair of ends 24, 26. Each ofthe pair of ends 24, 26 is connected to another folded strut member 25and also to the end of a flexible member 35. Thus, each end 34, 36 of aflexible member 35 is connected to two ends 24, 26 of a folded strut 25section member.

[0034] Each of the folded struts 25 takes on a generally irregularpattern. On the other hand, each of the flexible sections 35 takes on agenerally undulating pattern. The folded strut sections 20 wrapcircumferentially around the cylindrical shape of the stent 10. Eachflexible section 30 also connects to a folded strut section 20 aroundthe circumference of the stent. It will be noticed that each adjacentflexible section 30 is positioned 180° out of phase with each other.

[0035] The longitudinal lengths of the folded struts 20 are short enoughto give a smooth profile when the stent is bent. The folded strut 20allows for a large diametrical expansion range upon expansion. So, uponexpansion, the folded struts 20 expand circumferentially and becomehoop-like so that maximum radial strength is achieved. The flexiblemembers 30 placed between the folded struts improve the stentdeliverability in the unexpanded dimension of the stent 10. Theseflexible members are longitudinally compliant so that foreshortening isminimized upon expansion.

[0036] In use, therefore, the stent 10 of the present invention isplaced on a balloon catheter and is snaked through the vasculature to beplaced into a lesion site in an artery, typically a coronary artery.Because the flexible sections 30 are so substantially flexible, they areable to navigate tortuous lesions with relative ease. Once in place, theballoon catheter is expanded by conventional means. Upon expansion, thestruts 25 in the folded strut sections 20 expand to obtain a hoop-likeshape. In addition, these members expand longitudinally, so that anyreduction in foreshortening is negated. Of course, upon expansion, theflexible members 35 straighten so that there is further strengthachieved by the stent in the straightened and rigid positions.

[0037] A variation of the present invention can be seen in the stent 50of FIGS. 2 (“angled” version) and 3 (“straight” version). There, theradial strength sections 120 are achieved with generally straightmembers 115, although these members do not have folded struts.Connection between generally straight members 115 is made by connectingthe generally straight members 115 to the more flexible members 125,much like the connection made involving the connecting members of thefirst embodiment of FIG. 1.

[0038] The members that reduce foreshortening are angled members 130which are seen to be 180° out of phase with one another. The connectionbetween the flexible members is made at the end of a particularrelatively non-flexible member and at the distal end of a particularangled canted member 130. Now, when the columns comprised of relativelyrigid members 115 expand, the length of these members 130 shorten. But,the longitudinal lengths of the canted members 130 are placed at anangle compared to the longitudinal axis of the stent 50. So, uponexpansion, these canted members 130 actually lengthen with respect tothe longitudinal axis of the stent 50. The net result is that noforeshortening occurs upon expansion of stent 50.

[0039] The canted members 130 are angled in order to both: increaseflexibility; and to provide additional resistance on the balloonsurface. This arrangement helps prevent what is known as “dogboning” orexposure of leading edge of any of the strut members 75 contained ateither end of the stent 50. In addition, this configuration alsoprevents slippage of the stent along the balloon surface. The cantedmembers 130 are canted in opposite phase (i.e., with a phase change of180°) to one another, in order to negate any torsional effects on thestruts 75,85 along the length of the stent. These particular members canbe crimped to a lower profile than the more rigid members, in order toensure increased retention of the stent on the surface of a ballooncatheter. Further the configuration described herein has a uniquelyfolded configuration reducing any risk of “flaring” of the edges ofstruts 75, 85 during traversal of the lumen.

[0040] It is to be noticed that the longitudinal position (the “order”)of the columns can be changed if one desires a smaller initial profile.That is, if one desires that the profile be smaller, it is possible toremove the more rigid sections 120 (or a portion thereof,) and replacethem with the generally canted sections 130.

[0041] It is also to be noticed that the wave amplitudes of the strutsin a particular column are not kept constant. The wave amplitudes,defined herein as “W,” can be lengthened where permitted by thegeometry. For instance, notice the space S created between one set ofstrut members A and a second set of strut members B. This particularconfiguration allows an increased expansion range around the unexpandedcircumference of the stent, while maintaining an appropriate expansionarea associated with the metallic struts placed around of thecircumference of the stent. Such optimization of the strut surface areais important to ensure adequate coverage of the lesion upon expansion ofthe stent.

[0042] The stent 50 of this particular embodiment is expanded in muchthe same way as the stent 10 of FIG. 1. When expansion occurs via theballoon catheter, the canted members 130 tend to lengthen and preventforeshortening of the stent 50; the relatively rigid members 120 tend toshorten in the longitudinal direction, but in so doing provide a greaterrigidity for the fully expanded stent. It is to be understood however,that in the expansion of both stents 10, 50 the ability to flexiblynavigate the vasculature is enhanced from configuration of either stent10, 50, as the case may be. All the while, the likelihood of stentforeshortening upon expansion is greatly reduced.

[0043] As can be seen in FIG. 4, one can also provide for a stent 175that does not contain canted sections. Yet, the stent 175 expands withdecreased foreshortening along its length due to the unique geometry ofthe stent 175. Here, the stent struts 180, 190 provide for a relativelyconstant length along the longitudinal axis. (In other words, thelongitudinal dimension of the struts 180, 190 in combination remainsrelatively constant, whether in the expanded or unexpanded condition.)In this fashion, upon expansion, the stent 175 maintains a generallyconstant length in any of its expanded, unexpanded or partially expandedconditions.

[0044]FIGS. 4 and 5 show yet another embodiment of the design of asimilar stent 200. Here, the connector 250 is shaped like an “N,” muchafter the same fashion of “N”-shaped connectors found commercially inthe Bx Velocity.® stent sold by Cordis Corporation, Miami Lakes Fla. andwhich is at least somewhat characterized in Ser. No. 09/192,101, filedNov. 13, 2000, now U.S. Pat. No. 6,190,403 B1, and Ser. No. 09/636,071,filed Aug. 10, 2000, both of which are assigned to Cordis Corporation,and incorporated herein by reference.

[0045] In the stent 200, the relatively rigid sections R contain unequalstruts 210, 220 of lengths a, b, as can best be seen in FIG. 4.Moreover, as can be seen in FIG. 5, this strut pattern is formed so thatthe attachment points a at the end of the flexible connectors 250 can belocated at any point along the struts 210, 220 rigid section. In thisfashion, when the stent is expanded, the relatively more rigid section R“holds” the connector 250 along the surface of the lesion, so thattenacity of the stent, and its concomitant support are both maintainedto a high degree at the situs of the lesion. Yet, in the unexpandedconfiguration, the “N”-shaped flexible connectors 250 are able to guidethe stent 200 around the curvature of generally any tortuous vessel,including tortuous coronary arteries.

[0046] As can be seen from FIGS. 4 and 5, the alternative embodimentstent 200 is also capable of reducing foreshortening along its entirelength. This stent contains relatively rigid sections R and relativelyflexible sections F containing connectors 250. (The flexible sections Fare in the form of undulating longitudinal connectors 250.) Therelatively rigid sections R generally contain a slotted form, createdwith struts 210, 220 around a slot S. The relatively rigid sections Rcontain these interlaced struts 210, 220, which are of varyinglongitudinal dimensional length.

[0047] As can be seen from the figures, in some radial positions, thestruts 210 are made longer. In other radial positions, the struts 220are made shorter. However, the shorter struts 220 are of a constantlength b in the longitudinal dimension, and in the fashion in which theyconnect to the relatively flexible connectors 250. Also, as describedabove, the relatively more rigid sections R maintain the relatively moreflexible sections F at a generally constant longitudinal length due tothe friction maintained by the relatively more rigid sections R on aballoon portion of an angioplasty type balloon catheter. Accordingly,upon expansion, the constant length b, in conjunction with the generallyconstant length of the relatively flexible connector 250, causes thestent 200 to maintain a relatively constant longitudinal dimension L inany diameter to which it is expanded. As can be appreciated, themaintenance of a constant length is desirable from the perspective ofsecure, repeatable placement of the stent within the vasculature.

[0048] Continuing to describe the stent 200 of FIGS. 4 and 5, theflexible sections F operate with the behavior of the flexible connectors250 acting in the fashion of “N”-shaped flexible connectors of similartype. That is, the flexibility of the stent 200 is focused in this areaF so that one is able to traverse tighter lesions using such aconfiguration. The relatively stronger sections R are capable ofexpansion to a stronger plastically deformed dimension, so that in thisfashion the stent 200 is capable of supporting the arterial wall. Eventhough the longitudinal dimensions of the struts 210, 220 in therelatively stronger sections R are of unequal length, such aconfiguration does not diminish radial support in the expandedcondition. Accordingly, it can be appreciated that a stent of this shapewill adequately support the arterial walls at the lesion site, whilemaintaining radial flexibility, and longitudinal length.

[0049] As can be best seen in FIG. 7, yet another alternate embodimentof the present invention is described. In FIG. 7, there is contained astent 300 much like the Bx Velocity ® stent sold by Cordis Corporation,Miami Lakes, Fla. In FIG. 7 there is contained on the stent 300generally flexible connector members 310 connected to generally rigidradial strut members 320. The connector members 320 are generally formedin the shape of the letter “N”, and the struts 310 are generally slotsformed in a radial fashion around the circumference of the stent. Theconnection made between the flexible connectors 320 and the radial strutmembers 310 is formed from a living hinge 330. This living hinge 330contains outer radial arc 332 and an inner radial arc 334. In theexpanded configuration, the radial arcs 332, 334 move away one from theother, so that the overall length of the living hinge 330 actuallyincreases upon expansion.

[0050] Known conventional means, such as angioplasty balloons, or theballoon on a stent delivery system expands the stent 300 of the presentinvention. Upon expansion, there are provided a number of benefits bythe stent 300 of the present invention. First, as explained above, thereis reduced foreshortening of the stent 300, since the outer radial arc332 in fact does not foreshorten. Since it lengthens slightly, theoverall length of the stent 300 is maintained to its general nominallength. There is also provided increased radial strength since theradial arcs 332, 334 at their connection between the flexible and radialstruts 320, 310, (both inner and outer radial arcs 334, 332) combine togive superior strength in the arcs' section; the radial strut 310provides for optimal strength in the radial direction since it isparallel to the loading direction of the stent 300, thereby creating a“hoop” a circumference C of the stent. Also, because the radial arcs areable to accept greater forces, there is reduced strain for theequivalent strength designed for a stents. In all, the stent 300 of thisembodiment provides for at least equivalent radial strength, lessforeshortening and reduced strain when compared to current stents.

[0051] As can be seen from FIGS. 8, 9 and 10, there is provided yetanother embodiment of the stent 400 in the present invention. Again, thestent 400 provides for generally stronger radial sections R comprisingradial struts 410, which are generally slotted in alternating fashionaround the circumference of the stent. The flexible connector members420 are similar to the flexible connector members as seen in FIG. 7, andalso to the flexible connector members of the Bx Velocity ® stent.However, these flexible connector members 420 are connected to theradial struts generally somewhere near the midpoint of the radial struts410. In this fashion, upon expansion the length of the connector members420 remains independent of the shortening or lengthening of the radialstruts 410. In this way, the overall length of the stent is maintained,as seen from the schematics in FIGS. 9 and 10.

[0052] Due to this overall ability to maintain the length of stent 400,the radial struts 410 provide for radial strength only, and do notcontribute in one way or another to any foreshortening of the stent.Also, the radial struts 410 are formed from a generally “wavy” pattern.This wavy pattern is useful in helping to reduce the crimp profile ofthe stent 400 on the balloon. This results from the relative smoothattachment of the radial struts 410 to the flexible connectors 420.Further, having such an arrangement reduces the strain placed on thestruts 420 upon expansion. This reduced strain is achieved due to thelocation of the connection of the struts 420 to the struts 410. Becausethere is relatively little movement of the struts 420 in thelongitudinal direction, there is relatively little strain placed onthese struts during expansion. The radial arcs 415 of struts 410 can beideally placed in a “shifted” configuration so that the stent is easierto crimp on a balloon.

[0053] Further, this can be seen from FIG. 8, that the radial strutmembers 410 are attached to the flexible connectors 420 so that theflexible connectors 420 generally proceed along a “spiral” pattern Saround the length of the stent 400. The connection points 422 of theflexible connectors 420 are placed in a diagonal fashion on struts 410so as to enhance flexibility. Generally connectors 422 are located on amidpoint of a strut 410. When the connectors 422 are placed past themidpoint of strut 410 (i.e., farther from the midpoint of strut 410 thanfrom the direction of connector 420), the nominal stent strength shouldincrease upon expansion when compared to the above described stent. Thisarrangement reduces foreshortening, as described above. Further, thearrangement in no wise affects any torsion on the stent as it is appliedto the lumen by the balloon catheter. Friction of the balloon to struts410 maintains the struts 410 (and their opposite struts 420) in the samegeneral radial position throughout expansion. By reducing any concern ofstent torsion, there is also a reduced concern of overall slippage ofthe balloon. Even though the connector members 420 are not aligned withone another, they are maintained in their respective positions on theballoon surface, Upon expansion, struts 420 lock into place, as thestent 400 is placed, giving an increased strength in the lumen.

[0054] From FIGS. 8 and 9, we see that the midpoint of a connector 420is important to maintaining length. The greater the distance fromconnector 420 to the midpoint M, on the side of the connection betweenstruts 410, 420, the greater the potential for shortening of the stent.This creates a need to solve any shortening by other means, absent thesolution described herein.

[0055] It is to be understood that various modifications to the stent400 of FIGS. 8, 9 and 10 are possible without departure from theinvention herein. For instance, the connectors 420 can be placedintermittently about the stent 400 circumference, and not at everyincidence of a radial strut 410. Also, while the radial struts 410 aregenerally 900 out of phase between one series of struts 410 a and thenext 410 b, it is foreseeable to place them between 30° to 150° out ofplace. When so placed, the struts 410 can be “encouraged” to bend in aparticular fashion, which may be preferential in the design of aparticularly intended stent.

[0056] These stents can be manufactured by know conventional means, suchas laser etching, electrical discharge machining (EDM), photochemicaletching, etc. However, there is also disclosed in the invention herein anovel method of performing photochemical resistance etching of the tubefrom which the stent is to be made. This novel method allows one toproduce a stent with variable geometry in the three dimensions of thestrut, that is, along its length, across the circumferential dimension,and along its depth (or radial dimension.) This method starts with astandard photochemical machining process.

[0057] The new process consists of cutting the stent using photochemicaletching, cleaning it, and then coating it with a photoresist. Thephotoresist coating is applied in circular shapes 290, as can beappreciated from FIG. 6. These shapes 290 are intentionally figured tobe of varying dimension in their radius. Then, a photoresist image isdeveloped on the surface of the cylindrical metallic tube T from whichthe stent starts. This photoresist image is developed in a controlledfashion using known means. The development of the photoresist in thisfashion allows a controlled variable etching rate at select positionsalong the cylindrical metallic tube.

[0058] As previously stated, the novel photoresist image can be seen inFIG. 6. This photoresist image consists of a series of circular regionsof photoresist material 310, which are shaped in a variable diameter asdesired for manufacture. These photoresist images 310 are configured atvariable distances D from one another. As the diameter of the circularphotoresist pattern 310 decreases, and its distance from otherphotoresist patterns 310 increases, the etching rate of that area of thestent increases. Thus, by strategically placing the photoresist patterns310 on the stent, one can produce any variable dimension in anydirection along the stent.

[0059] This photoresist pattern 310 variation results in a variation inthe metal of the stent removed during the etching process. This processcan be used to locally change the geometry of the metallic tube.

[0060] In this fashion, one can envision making a stent of variablecircumferential width, radial depth or longitudinal length. As such, onecan impart varying flexibilities along the stent longitude, as well asvarying strengths so that a stent can be configured for emplacement atvarious locations within the body.

What is claimed is:
 1. A stent having a generally tubular configurationand a longitudinal axis, comprising: a radial strength portion and aflexible portion, each of said radial strength portion comprisingplurality of radial struts and said flexible portion comprising aplurality of flexible struts, and a flexible strut connected to a radialstrut along the length of a radial strut closer to the midpoint of theradial strut than to the end of the radial strut.
 2. A stent having agenerally tubular configuration and a longitudinal axis, comprising: aradial strength portion and a flexible portion, each of said radialstrength portion comprising plurality of radial struts and said flexibleportion comprising a plurality of flexible struts, and a flexible strutconnected to a radial strut along the length of a radial strut at themidpoint of the radial strut.
 3. The stent of claim 1 wherein a seriesof flexible struts forms a spiral shape around the circumference of thestent.
 4. The stent of claim 2 wherein a series of flexible struts formsa spiral shape around the circumference of the stent.
 5. The stent ofclaim 1 wherein the a pair of adjacent radial struts in a ring of radialstruts is placed about 90° out of phase with the pair of radial strutsclosest to it in an adjacent ring of radial struts.
 6. The stent ofclaim 2 wherein the a pair of adjacent radial struts in a ring of radialstruts is placed about 90° out of phase with the pair of radial strutsclosest to it in an adjacent ring of radial struts.